Nocturnus - I don't know too much about them. They released "The Key" in 1991, and it's one of the first (probably not THE first..) death metal albums to incorporate keyboards. It's quite good. Lyrics are kinda science fiction based, vocals are very raspy.
Pestilence - I just bought "Testimony of the Ancients" and it kicks major ass. Very technical, yet still retaining memorable hooks and riffs. I don't know how it compares to their other albums, but from reading the other responses and the poll that was posted here a while back, you really couldn't go wrong with any of their albums.
Atheist - I've only heard a handful of songs from them. Pretty good if you like the death/jazz combination.